The increasing need for data communication services is being accommodated today by using both separate data networks and by integrating digital data communication service over existing voice communication (telephone) networks. Typically, when both voice and digital data communication capabilities are provided over the existing telephone network a modem is utilized to provide the digital data communications. In such systems, the digital data is converted to an analog signal using the modems and no changes are required at the local telephone central office to provide a digital data communication channel. However, when it is desirable to transmit high speed data as a digital signal, circuit changes are required at both the user's location and at the local telephone office to enable the transmission and reception of the digital signal over the loop facility. Additionally, if it is desirable to have a system with a capability to transmit either voice or digital signals over the loop facility, the user must signal the operating mode of the system to the local telephone office.
In one prior signaling method the customer signal changes in the system operating mode to the local office by dialing special codes and awaiting a confirmation signal from the local central office. In such an arrangement, the operating mode signaling from the customer to the local office exists as either frequency tones or digital data characters. When frequency tones are used for mode signaling a tone receiver is required at the local office. Similarly, if digital data characters are used for mode signaling, a digital data receiver is required at the local office. Both the frequency tone mode signaling and digital character signaling arrangements require using the same loop facility used for the transmission of the voice or digital data information. To prevent interference with the voice or digital data signals mode, signaling must be done at a time when no voice or digital data signals are on the loop facility. Consequently, since the system operating mode information is not always present on the loop facility, system memory must be utilized to remember the system operating mode. Additionally, this memory must be accessed each time the operating mode information is required by the telephone network. The above tone or digital character signaling arrangements result in additional complexity in the circuitry of a shared voice/digital data system which share a common telephone network.